.TH PTRACE 2 "September 27, 2009"
.UC 4
.SH NAME
ptrace \- process trace
.SH SYNOPSIS
.nf
.ft B
#include <sys/types.h>
#include <sys/ptrace.h>

long ptrace(int \fIreq\fP, pid_t \fIpid\fP, long \fIaddr\fP, long \fIdata\fP)
.ft R
.fi
.SH DESCRIPTION
The \fBptrace\fP call provides a primitive means to trace (debug) another
process. A process can submit itself to tracing using a \fBT_OK\fP ptrace
request, or can be attached to by a tracer using a \fBT_ATTACH\fP request.
From that point on, whenever a signal is sent to the traced process,
the process will be stopped. Its tracer will be told about the signal
causing the stop, via
.BR wait (2).
The tracer can then inspect the traced process, and choose how to continue the
process's execution and whether to pass on the signal to it.
.PP
In the current model, the tracer will be notified of the signal before any
checks on ignore or block masks are made. A \fBSIGKILL\fP signal cannot be
intercepted by the tracer, and will always kill the traced process.
.PP
When the traced process performs a successful
.BR execve (2)
call, it will be stopped and a \fBSIGTRAP\fP will be generated for it.
Set-uid and set-gid bits on the new executable are ignored.
.PP
The \fIreq\fP parameter specifies the process trace request. The interpretation
of the remaining parameters depends on the given request. For all requests
except \fBT_OK\fP, the \fIpid\fP parameter specifies process ID of the target
process. For all requests except \fBT_OK\fP and \fBT_ATTACH\fP, the process
must be stopped. The following requests are supported:
.TP 2
.B T_OK
Set the caller's parent to be its tracer. All other arguments are ignored.
This request is typically made by the child fork of a debugger,
before performing an
.BR execve (2)
call.
.TP
.B T_GETINS, T_GETDATA
Retrieve a value from the given process's instruction and data area,
respectively, at the address given in \fIaddr\fP.
.TP
.B T_SETINS, T_SETDATA
Set the value from the given process's instruction and data area, respectively,
at the address given in \fIaddr\fP, to the value given in \fIdata\fP.
.TP
.B T_GETUSER
Retrieve the value at the zero-based offset given in \fIaddr\fP from the
process's \fBstruct proc\fP kernel structure, followed by, aligned on
\fBlong\fP size boundary, its \fBstruct priv\fP kernel structure.
.TP
.B T_SETUSER
Set some of the given process's registers at the beginning of its
\fBstruct proc\fP kernel structure. The value in \fIdata\fP will be written to
the zero-based offset given in \fIaddr\fP from the process's \fBstruct proc\fP
kernel structure.
.TP
.B T_RESUME
Resume execution of the process. A nonzero \fIdata\fP argument will be
interpreted as a signal to pass to the process.
.TP
.B T_STEP
Single-step an instruction. A nonzero \fIdata\fP argument will be interpreted
as a signal to pass to the process.
.TP
.B T_SYSCALL
Resume execution with system call tracing. When the traced process makes a
system call, a \fBSIGTRAP\fP signal will be generated. A subsequent
\fBT_SYSCALL\fP request will then cause a \fBSIGTRAP\fP signal to be generated
when the process leaves the system call. A nonzero \fIdata\fP argument will be
interpreted as a signal to pass to the process.
.TP
.B T_EXIT
Terminate the traced process, with the exit code given in the \fIdata\fP
argument. This call will return once the process has exited.
.TP
.B T_ATTACH
Attach to the given process. The process will be stopped with a \fBSIGSTOP\fP
signal.
.TP
.B T_DETACH
Detach from the given process. Any signals still pending for the tracer are
passed on directly to the process. A nonzero \fIdata\fP argument will be
interpreted as an additional signal to pass to the process.
.TP
.B T_SETOPT
Set the given process's trace options to the bit combination of flags given
in the \fIdata\fP argument.
.TP
.B T_GETRANGE
Get a range of values from the address space of the traced process. The
\fIaddr\fP argument must be a pointer to a fully initialized
\fBstruct ptrace_range\fP structure.
.TP
.B T_SETRANGE
Set a range of values in the address space of the traced process. The
\fIaddr\fP argument must be a pointer to a fully initialized
\fBstruct ptrace_range\fP structure.
.PP
The following option flags are currently supported for \fBT_SETOPT\fP:
.TP 2
.B TO_TRACEFORK
When the traced process performs a
.BR fork (2),
automatically attach to the new child as well.
The child will be stopped with a \fBSIGSTOP\fP signal right after forking.
.TP
.B TO_ALTEXEC
Send \fBSIGSTOP\fP instead of \fBSIGTRAP\fP upon a successful
.BR execve (2).
This allows the tracer to disambiguate between this case and other traps.
.TP
.B TO_NOEXEC
Do not send any signal upon a successful
.BR execve (2).
.PP
The default set of trace options when tracing is initiated with \fBT_OK\fP is
\fB0\fP.
The default set of trace options after attaching with \fBT_ATTACH\fP is
\fBTO_NOEXEC\fP.
.PP
The \fBT_GETRANGE\fP and \fBT_SETRANGE\fP calls use the following structure:
.PP
.RS
.nf
.ft B
.ta +4n +8n
struct ptrace_range {
	int	pr_space;
	long	pr_addr;
	size_t	pr_size;
	void	*pr_ptr;
};
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.fi
.RE
.PP
The \fBpr_space\fP field specifies the address space from which to retrieve
or set the values. It must be set to either of the following values:
.PP
.TP 10
.B TS_INS
Text space.
.TP
.B TS_DATA
Data space.
.PP
The \fBpr_addr\fP field specifies the start address of the target area in the
traced process. The \fBpr_size\fP field specifies the number of bytes to
retrieve or set, and must be between 1 and LONG_MAX. The \fBpr_ptr\fP field
must point to a buffer in the calling process that is used to store the
retrieved values (for \fBT_GETRANGE\fP) or contains the values to set (for
\fBT_SETRANGE\fP).
.PP
All addresses specified for the \fBT_GETINS\fP, \fBT_GETDATA\fP,
\fBT_GETUSER\fP requests and their \fBT_SET\fP* counterparts must be
aligned on \fBlong\fP boundary. Similarly, only \fBlong\fP sized values can be
retrieved and set at a time.
.SH "RETURN VALUE"
All but the \fBT_GETINS\fP, \fBT_GETDATA\fP, \fBT_GETUSER\fP requests return 0
upon successful completion.
Otherwise, a value of -1 is returned and \fIerrno\fP is set to indicate the
error.
.PP
The \fBT_GETINS\fP, \fBT_GETDATA\fP, \fBT_GETUSER\fP requests return the
resulting data. Here, -1 is a legitimate return value.
To distinguish between this and an error, clear \fIerrno\fP
before the \fBptrace\fP call, and check whether it is zero afterwards.
.SH ERRORS
The functions will fail if any of the following errors occur:
.TP 10
.B EINVAL
Invalid request, signal, space, or length given.
.TP
.B ESRCH
The given process is not found, exiting, or not traced by the caller.
.TP
.B EBUSY
The given process is not stopped, or already being traced.
.TP
.B EFAULT
The given address is invalid, inaccessible, or not properly aligned.
.TP
.B EPERM
Attaching is denied, because the caller equals the given process,
or the caller is not root and does not match the given process's
user or group ID, or the caller is not root and the given process
is a system process, or the caller is a system process,
or the given process may not be traced at all.
.SH LIMITATIONS
Signals are not ordered. Attaching to a process guarantees that a \fBSIGSTOP\fP
will arrive at the tracer, but it is not guaranteed that this will be the first
signal to arrive. The same goes for automatically attached children of the
traced process. Similarly, if the tracer wants to detach from a running
process, it will typically send a \fBSIGSTOP\fP using
.BR kill (2)
to the process to stop it, but there is no guarantee that this will be the
first signal to arrive.
.PP
Signals not caused by the process itself (e.g. those caused by
.BR kill (2))
will arrive at the tracer while the process is in stopped state, but this does
not imply that the process is in a stable state at that point. The process may
still have a system call pending, and this means that registers and memory of
the process may change almost arbitrarily after the tracer has been told about
the arrival of the current signal. Implementers of debuggers are advised to
make minimal assumptions about the conditions of the process when an unexpected
signal arrives.
.PP
It is not possible to use \fBT_SYSCALL\fP to get a trap upon leaving of a
system call, if \fBT_SYSCALL\fP was not used to get a trap upon entering that
system call. This is in fact helpful: after attaching to a process, the first
\fBT_SYSCALL\fP call will always cause a trap after entering the next system
call. As the only exception, \fBT_SYSCALL\fP on a
.BR fork (2)
call of a process with \fBTO_TRACEFORK\fP set, will result in two traps upon
leaving: one for the parent, and one for the child. The child's \fBSIGSTOP\fP
signal will always come before the \fBSIGTRAP\fP from its leaving the system
call.
.PP
There is no way to reliably distinguish between real signals and signals
generated for the tracer.
.PP
For system stability reasons, the PM and VM servers cannot be traced.
.SH "SEE ALSO"
.BR wait (2),
.BR kill (2),
.BR mdb (1)
.SH AUTHOR
Manual page written by David van Moolenbroek <dcvmoole@cs.vu.nl>
